The present invention relates to a holder for supporting wafers to be processed which is installed in an ion implantation apparatus used in the manufacturing process of semiconductor devices and, more particularly, to a wafer holder having a electrically conductive elastic body that is installed in an ion implantation apparatus to ensure excellent workability and enhancement of wafer cooling efficiency.
In general an ion implantation apparatus is comprised of a beam line part wherein ion beams are generated and an ion implantation part wherein the silicon wafers to be processed are irradiated with ion beams from the beam line part to undergo ion implantation. As to the ion implantation part, an ion implanting part having a target chamber (vacuum chamber) and a wafer holding wheel installed in the target chamber is widely known.
Such a wafer holding wheel is constituted of a hub installed in a target chamber so that it can rotate with a rocking motion and arms radiating from the hub in all directions, and the tip of these arms each is equipped with a wafer holder for holding a silicon wafer.
In this case, the heat generates upon ion beam-shooting of wafers carried out in the ion implantation apparatus to cause deterioration in the photoresist layer formed on each wafer. For preventing such deterioration, it is necessary to cool the wafers to 100xc2x0 C. or below.
Therefore, it has generally been carried out to cool the wafer holding wheel by circulating a coolant therein. However, the thermal conductivity at the interface between the wafer and the wafer holder shows a sharp drop in a vacuum, so that effective cooling of wafers cannot be achieved merely by circulating a coolant in a wafer holding wheel.
For the purpose of improving the physical contact between the wafer and the wafer holder, the use of a thermally conductive polymer on the wafer holder surface has been proposed, and carried into actual practice. For instance, the inert polymer thin film having tackiness is disclosed in U.S. Pat. No. 4,139,051, and the thermally conductive silicone rubber layer is disclosed in U.S. Pat. No. 4,282,924.
As to the method for supporting a wafer on a wafer holder, on the other hand, the method of fastening a wafer to a wafer holder with a ring laid on the periphery of the upper surface of the wafer has hitherto been put to practical use. However, such a method has a drawback of making it impossible to use the periphery of the wafer for a semiconductor device.
As another supporting method developed with the intention of giving up laying a fastening ring on the periphery of each wafer in order to use the whole surface of the wafer, centrifugal fastening has been put to practical use. In this method, each wafer revolves around a pivot as it is held on a wafer holder. And the angle between the pivot and each wafer holder surface is adjusted so that each wafer is pressed tightly against the wafer holder by the centrifugal force.
The art of utilizing both the foregoing centrifugal fastening method and thermally conductive polymer layer is disclosed in U.S. Pat. No. 4,832,781, and it is highly effective in holding and cooling wafers.
However, the use of thermally conductive silicone rubber for the foregoing thermally conductive polymer layer creates two problems. The first problem is that after ion implantation the wafer is stuck to the thermally silicone rubber surface and will not come loose; as a result, the wafer breaks if it is peeled away by force.
A cause of such tight adhesion of a silicon wafer to an elastic body is thought to be due to an electrostatic adsorption phenomenon arising from charges accumulated on the wafers by ion implantation.
The second problem is as follows: The elastic bodies according to conventional arts have an even surface, so that the wafer is mounted on such an elastic body in a state that almost all the back surface of the wafer is in close contact with the elastic body. When the wafer holding wheel is rotated at a high speed in order to implant ions into wafers, the centrifugal force generated thereby causes the slippage of the wafer on the elastic body; as a result, the elastic body is abraded to form particles, and these particles adhere to the back surface of the wafer.
Considering that improvements in yield and throughput are important to the semiconductor manufacturing process, the aforementioned first problem is a big problem, so the solution thereof has been desired.
The second problem described above gives rise to an apprehension that the cleaning tank used in the cleaning step subsequent to the ion implantation step is polluted with the particles or the particles on the back side of a wafer move around to the front side of the wafer.
Therefore, a first object of the invention is to provide a wafer holder installed in an ion implantation apparatus, which enables sufficient reduction of the heat generating in the wafer during the ion implanting operation to protect the resist layer of the wafer and ensures consistent operation in the apparatus by preventing the wafer from tightly contacting with the elastic body surface by electrostatic adsorption arising from charges accumulated on the wafer during the ion implantation.
A second object of the invention is to provide a wafer holder installed in an ion implantation apparatus, which can decrease the number of particles adhering to a wafer when the wafer is mounted on the surface of an elastic body.
The aforementioned objects of the invention are attained with wafer holders which are disposed in the target chamber of an ion implantation apparatus, with the wafer holders each comprising a wafer mount and an electrically conductive elastic body that is laid on the wafer mount and has a surface for holding a wafer thereon, wherein the surface of the elastic body preferably has a shape enabling a partial contact with the wafer held thereon.